LED devices made of semiconductor crystal are widely used as display elements. These LED devices are mostly made of II-V compound semiconductors. III-V compound Semiconductors have a band gap corresponding to the wavelength of visible light and infrared light and thus have been applied to light emitting elements. Among these III-V compound semiconductors, GaAsP has been in great demand as an LED. Since the most important characteristics of GaAsP LED devices are light emitting power and life, improvements in these characteristics have been desired.
GaAs.sub.1-x P.sub.x (0.45&lt;x&lt;1) is doped with nitrogen (N) as an isoelectronic trap to thereby enhance light emitting efficiency. This LED device exhibits a light output enhanced about 10 times. In general, this LED device can be stably obtained by a process which comprises vapor phase epitaxial growth of N-type layers using a quartz reactor, and then diffusion of zinc from the surface of a light emitting layer to form a P-type layer therein, thereby forming a PN-junction.
FIG. 4 illustrates a general structure of a GaAsP epitaxial wafer.
In FIG. 4, the GaAsP epitaxial wafer is made of a GaP single crystal substrate 1, on which a homo-layer 2 having the same composition as that of the substrate 1, a GaAs.sub.1-x P.sub.x graded composition layer 3 having a composition continuously changing from 1.0 to x.sub.0 to relax the lattice mismatch between the substrate and the uppermost layer, a GaAs.sub.1-x0 P.sub.x0 constant composition layer 4, and a GaAs.sub.1-x0 P.sub.x0 N-doped layer 5 doped with nitrogen are formed in this order. The uppermost layer of the epitaxial wafer is a light-emitting layer having a constant composition x.sub.0, which is arranged to obtain the desired emission wavelength of the LED. This light emitting layer has been doped with nitrogen and tellurium (Te) or sulfur (S) in such an amount that a predetermined carrier concentration is realized. Usually, the composition x.sub.0 is about 0.65 for emission of red light (wavelength: 630 nm). Nitrogen acts as an isoelectronic trap which serves as a light emitting center. However, nitrogen is electrically inactive and thus does not contribute to the carrier concentration.
In order to put this LED material into practical use as an LED device, it is necessary that the electrical characteristics of this LED material be improved. JP-A-53-64488 (The term "JP-A" used herein means an "unexamined published Japanese patent application") discloses a structure having a carrier concentration gradient in which the carrier concentration is from 1 to 20.times.10.sup.16 only in the portion that serves as a light emitting layer, and but not less than 10.times.10.sup.16 in the other epitaxial layers. The above-cited patent describes that the thickness of the GaAsP epitaxial layer in its attached drawings is 100 .mu.m at one end of the wafer and 230 .mu.m at the other.
It is also known that the carrier concentration should be from 3.5 to 8.8.times.10.sup.15 cm.sup.-3 to minimize the destruction of the completeness in crystallinity of the light emitting layer during the formation of the PN-junction by the diffusion of zinc from in the surface of the light emitting layer. This prolongs the life of carriers thus injected, resulting in an LED device having a high light output (JP-A-55-9467). It is further known that when the carrier concentration of the light emitting layer is further reduced to not more than 3.times.10.sup.15 cm.sup.-3, the enhancement of light output and the prolongation of LED life can be realized at the same time as shown in FIGS. 5 and 6. It is further known that the layers other than the light emitting layer are droped with Te or S to a carrier concentration as high as from 0.5 to 5.times.10.sup.17 cm.sup.-3, to thereby reduce the electrical resistance of the LED (JP-A-6-196756).